1. Field of the Invention
The present invention relates generally to fiber optic sensors, and more particularly to a method and apparatus for independently measuring the temperature and axial strain of an optical fiber.
2. Technical Background
Fiber optic sensors, and in particular distributed fiber sensors, are of interest for smart structures and other monitoring applications. Smart structures are often composite structures that may incorporate electrical communication devices for monitoring or actively controlling the operation of the structure. A sensor is required to monitor the conditions the smart structure is subjected to. A fiber sensor, for example, can be embedded within the layers of the composite structure to determine strain and temperature. There are other industrial applications that require knowledge of the environment in order to control both the quality and productivity of the process. Interest has peaked recently with the encouraging results obtained using Bragg gratings distributed along the length of the sensing fiber. One issue that arises with fiber optic sensors relates to their sensitivity to both temperature and strain. In one approach that has been considered, a combined strain and temperature sensor using polarization-maintaining fibers was developed. Unfortunately, it was determined that the temperature and the strain values obtained by the sensor were dependent upon one another. Thus, the values measured by the sensor were inherently skewed.
A sensor that can measure temperature without being adversely affected by a strain component, or conversely, a sensor that is able to measure strain without a temperature component is therefore desired.
In another approach, a first polarization-maintaining fiber having an elliptical core is fused to a second polarization-maintaining fiber having an elliptical core. The major axis of the second fiber is rotated 90.degree. with respect to the first fiber. When a polarized light signal is transmitted through the fibers, the temperature and strain affect the phase of the light signal differently. This relationship is characterized by the following equations: EQU .DELTA..phi..sub.1 =A.sub.1 L.sub.1.DELTA.T+B.sub.1.DELTA.L.sub.1, (1) EQU .DELTA..phi..sub.2 =A.sub.2 L.sub.2.DELTA.T+B.sub.2.DELTA.L.sub.2. (2)
wherein .DELTA..phi..sub.1 is the change in phase difference in the first fiber, A.sub.1 is the temperature coefficient for the change in temperature of the first fiber, L.sub.1 is the length of the first fiber, .DELTA.T is the change in temperature, B.sub.1 is the strain coefficient for the change in strain of the first fiber, .DELTA.L.sub.1 is the change in the length of the first fiber due to strain, .DELTA..phi..sub.2 is the change in phase difference in the second fiber, A.sub.2 is the temperature coefficient for the change in temperature of the first fiber, L.sub.2 is the length of the second fiber, .DELTA.T is the change in temperature, B.sub.2 is the strain coefficient for the change in strain of the second fiber, .DELTA.L.sub.2 is the change in the length of the second fiber due to strain.
In order to "de-couple" temperature and strain, the two fibers must be selected such that either their strain coefficients are equal, or that their temperature coefficients are equal, such that: EQU B.sub.1.DELTA.L.sub.1 =B.sub.2.DELTA.L.sub.2, or (3) EQU A.sub.1 L.sub.1.DELTA.T=A.sub.2 L.sub.2.DELTA.T. (4)
Thus, when the phase differences of the two fibers are subtracted, EQU .DELTA..phi.=.DELTA..phi.1-.DELTA..phi.2 (5)
The variable having equal coefficients is eliminated. Thus, a single variable is obtained. However, there are disadvantages to this approach. First, the two have fibers must be precisely selected to equalize the phase difference between the first and second fibers caused by either strain or temperature. Secondly, it is understood from equations 3, 4, and 5 that the sensor is limited to detecting either temperature or strain. It cannot detect both simultaneously.
Thus, a need exists for a fiber optic sensor that has the ability to accurately measure strain on a fiber without that measurement being affected by the temperature, while simultaneously being able to accurately measure the temperature of the fiber's environment without the temperature measurement being affected by the applied strain.